With regard to the stress measurement, the strain gauge is the commonly used device. However, the stress gauge only can measure a single point of a testing object. If the size of such testing object is too large, the installing time is lengthy. If this testing object is too small, it is hard to install it. This stress measurement cannot detect the distribution condition of internal thermal stress or flowing stress inside the testing object. Thus, the traditional measuring method is not convenient. But, if someone uses the photoelasticity measuring method, the above disadvantages can be solved.
The principle of the photoelasticity is based on the property of birefringence. When a light passes through a polarized plate, the vibration direction is limited to certain direction. Then, this polarized light passes through a birefringent material (that has the property of double refraction or birefringence) generating two lights with different velocities. Hence, its stress distribution (many lines) can be seen so that the user can determine the entire stress distribution of this testing object. These lines include isochromatics and isoclinies. The isochromatics are colored lines caused by the difference stress. Usually, it is determined by so-called “fringe order.” The isoclinics are to be observed to find out the developing direction of the principle stress.
The traditional retardation optical device 90 (as shown in FIG. 1) is used to observe the photoelastic lines and then the entire stress distribution can be seen. When the light X is white, the generating lines (such as the corresponding isochromatics) will be colored. If the light X is a monochrome, the generating lines (such as the corresponding isochromatics) will become black and white. When the isochromatics are colored, the user can observe the “fringe order” from the isochromatics, according to a stress-and-isochromatics converting table. When the isochromatics are black and white, the user can observe the distribution density of these lines (which is the isochromatics). If the distribution density is high, it means the stress concentration occurs.
However, no matter colored lines or black-and-white lines, there is a blurred zone, such as the blurred gradient zone between the purple line and blue line or the gray zone between the black line and white line. As illustrated in FIG. 2, the traditional retardation optical device 90 utilizes a micrometer to adjust the total thickness of the compensating plate (including two plates with a tilted sliding interface). The combined structure of the micrometer and the compensating plate is labeled by 91. By adjusting the thickness of the compensating plate, the retardation amount can be altered, so that the color of the lines in the blurred zone can be changed. It is easier to observe. However, the user has to change the thickness of the compensating plate (controlled by the micrometer) for different requirements. When the user observe a testing object 92, this one might need to change the thickness of the compensating plate many times for obtaining different retardation amounts. For example, if the user needs to change five different retardation amounts, this person has to change the thickness of the compensating plate five times (either by adjusting the micrometer or by replacing thicker or thinner plates instead). Therefore, the practical operation of the traditional device is time-consuming and inconvenient.